Lexicographers monitor the media for new words and new meanings, they try to sort out how words are used once there are enough examples. Then they have to decide if it's a word that's going to last and is widespread enough for inclusion in more than their database.
However, for the purposes of the law the legislators get to define the terms just like any other small speech community does. That's called a "jargon" or a "cant" and the meanings don't always show up in general-use dictionaries. Take the word "grammar." Most people stop at how words are put together. Some include how words are changed to fit into a sentence. I include word derivation and even sound changes and patterns. But that's linguistic jargon.
Legislators need to use standard definitions from case law, the dictionary, or from other legislation or simply impose their own definitions. They all work. Otherwise there's no good way to know what the law actually means. In the last 50 years, IMHO, no well-crafted law has been written that didn't include explicit definitions, with examples and limitations. Many poorly crafted laws have been written. Often intentionally.
Shifting definitions mid-argument, however, is a classic fallacy. Intentionally opening the door and expecting such a shift to happen in avoid to avoid the heavy lifting of legislating is, IMHO, a show of bad faith.
However, there's a long tradition of using courts (and in many cases regulators) to expand and shift definitions. In one case--was it MA--gay-marriage foes were incensed by precisely this. An amendment passed that used words that weren't defined but which had commonly accepted meanings--and opponents could see how eventually those words might be redefined to not only allow but require gay marriage. Those proposing the amendment countered opponents by saying they'd never push for such a redefinition. They didn't. Rather less than a decade later others did, and got the courts to redefine the terms. After all, just like the Constitution laws are also "living." (Redefinition is the heart of how the "living Constitution" actually "grows".) Another recent example is expanding "pollutant" to include carbon dioxide. Notoriously some government agency under Bush (I?) redefined ketchup as a vegetable.
Sounds like you're a devout originalist when it comes to laws that you don't want expanded and shifted.
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